Topic 6 of 9
Esters: join two fragments and keep track of their origins
The name has an alcohol part followed by an acid part.
O-Level 6092 (2026) / SEC G3 K324 (2027)
Pure onlyEsters: join two fragments and keep track of their origins
The name has an alcohol part followed by an acid part.
Warm a carboxylic acid with an alcohol and a little concentrated sulfuric acid catalyst to form an ester and water. The reaction is reversible. Ethanoic acid and ethanol give ethyl ethanoate: CH3COOH + HOCH2CH3 reversibly form CH3COOCH2CH3 + H2O. The ester linkage is -C(=O)-O-.
| Starting family | C1 | C2 | C3 | C4 |
|---|---|---|---|---|
| Unbranched acid part | Methanoate | Ethanoate | Propanoate | Butanoate |
| Alcohol part, -OH at chain end | Methyl | Ethyl | Propyl | Butyl |
The acid contributes the part ending in -oate, including its C=O carbon. The alcohol contributes the group attached through the ester oxygen. For an alcohol with -OH on a middle carbon, retain that attachment position when drawing the product; a numbered group such as propan-2-yl may be used when naming it.
Worked example
Name and reverse an ester
Identify the acid and alcohol that form CH3CH2COOCH3.
- Split at the bond between the single-bonded O and the alcohol-derived carbon group.
- CH3CH2CO- is the three-carbon acid fragment: propanoic acid.
- -CH3 attached to the ester O comes from methanol.
Methyl propanoate, made from propanoic acid and methanol. Count the carbonyl carbon when counting the acid carbons.